U.S. Anti-Doping Agency Chief Executive Travis Tygart said late Thursday he was still waiting to hear directly from Armstrong but added that the cyclist's decision not to proceed in an arbitration process will leave Armstrong stripped of all of his Tour titles and 2000 Olympic bronze medal and result in a lifetime competition ban.

"There comes a point in every man's life when he has to say, 'Enough is enough,'" Armstrong, 40, wrote in a statement emailed to The Times and other news agencies.

"For me, that time is now. I have been dealing with claims that I cheated and had an unfair advantage in winning my seven Tours since 1999."

Armstrong's attorneys asked a USADA attorney to turn the matter over to UCI, the international cycling union, but USADA maintains it retains jurisdiction to strip the titles.

Armstrong never tested positive for performance-enhancing use during his decade-plus of Tour races.

Now, as he abandons his impassioned fight against anti-doping authorities, the perception of an American hero who rallied from cancer to become champion of perhaps sport's most demanding endurance test has been recast.

Armstrong won the Tour de France from 1999 to 2005, but he was dogged by accusations from former teammates, including Floyd Landis — who was stripped of a Tour victory — and American rider Tyler Hamiltonthat his victories came amid team-wide deceit about performance-enhancing methods, including blood doping and steroid use.

"It is a sad day for all of us who love sport and our athletic heroes," Tygart wrote in a statement. "This is a heartbreaking example of how the win-at-all-costs culture of sport, if left unchecked, will overtake fair, safe and honest competition. For clean athletes, it is a reassuring reminder that there is hope for future generations to compete on a level playing field without the use of performance-enhancing drugs."

The U.S. attorney's office in Los Angeles earlier this year closed an inquiry without filing charges.

USADA, however, moved earlier this year to revoke Armstrong's Tour victories and was preparing to reveal details indicating doping by the cyclist while also calling witnesses before an independent arbitrator.

"Over the past three years, I have been subjected to a two-year federal criminal investigation followed by Travis Tygart's unconstitutional witch hunt," Armstrong wrote. "The toll this has taken on my family, and my work for our [cancer] foundation and on me leads me to where I am today — finished with this nonsense."

Armstrong said a federal court's decision this week not to halt USADA's review clinched his decision.

"If I thought for one moment that by participating in USADA's process, I could confront these allegations in a fair setting and — once and for all — put these charges to rest, I would jump at the chance," Armstrong said in the statement. "But I refuse to participate in a process that is so one-sided and unfair."

Armstrong has railed against the idea that a rider involved with doping can cut a favorable deal with USADA in exchange for testimony against him.

"Regardless of what Travis Tygart says, there is zero physical evidence to support his outlandish and heinous claims. The only physical evidence here is the hundreds of controls I have passed with flying colors. I made myself available around the clock and around the world. In-competition.... What is the point of all this testing if, in the end, USADA will not stand by it?"

Yet the federal court ruling and USADA's mountainous case also clearly backed Armstrong into a corner.

Betsy Andreu, who is the wife of a former Armstrong teammate and who told federal investigators she heard Armstrong admit to prior performance-enhancing drug use in 1996 when he was being treated for testicular cancer, said his decision to drop the fight closes a "dark era for the sport."

"Lance's story … is a cop-out; he is afraid of the overwhelming evidence against him to be presented in a public courtroom," Andreu said in an email to The Times. "… It is a very hopeful day for athletes who want to compete with integrity."

Armstrong sees the situation differently.

"I played by the rules that were put in place by the UCI … and USADA when I raced," he wrote. "I know who won those seven Tours, my teammates know who won those seven Tours, and everyone I competed against knows who won those seven Tours."